1. Field of the Invention
The present invention relates to a direct fuel injection ignition engine in which a cavity is formed on a top of a piston, in particular, to a configuration of the cavity of the direct fuel injection ignition engine.
2. Related Art
Conventionally, it has been known a direct fuel injection ignition engine an ignition plug is disposed along substantially a cylinder axis line of a combustion chamber and that an injector is disposed at a peripheral portion of the combustion chamber so as to inject a fuel directly into the combustion chamber. In this type of engine, for example, a fuel from the injector is injected obliquely downwardly toward the piston top at a compression stroke when the engine is in a low engine load and speed operating area so as to concentrate a combustion mixture around the ignition plug to accomplish a stratification combustion. As a result, a mean air fuel ratio is greatly increased or the combustion mixture is largely made lean.
In order to facilitate the stratified combustion, for example, as shown in Japanese Utility Model Registration No. 2527653, it is proposed that a cavity is formed on the top surface of the piston offset toward the injector (or intake port) and a projected portion projected toward a lower surface of the cylinder head is formed close to the opposite side of the injector or to the exhaust port.
With this structure, the injected fuel from the injector is trapped by the cavity and is facilitated to be vaporized by means of a swirl produced in the combustion chamber and introduced around the ignition plug to thereby accomplish a desirable stratification of the combustion mixture.
However, the conventional engine is disadvantageous in that the desirable stratification cannot be accomplished unless the cavity is properly formed.
For example, where the size of the cavity is too small in the axis direction of the cylinder, the injected fuel is excessively curved due to the swirl in a extreme low engine load operating area so that most of the injected fuel is diverted outside of the cavity and thus is not trapped by the cavity. As a result, the air fuel ratio around the ignition plug is undesirably increased or an undesirable lean combustion mixture is produced around the ignition plug. It is necessary to advance the fuel ignition timing in a relatively high engine load operating condition where the stratified combustion is executed. In this case, the fuel is injected under a low pressure in the combustion chamber so that the injected fuel enhanced in its injection force to be forwarded straightly without being influenced by the swirl. Thus, most of the injected fuel is trapped by the cavity. If the size of the cavity is too large in the axis direction of the cylinder, the injected fuel is not dispersed enough in the cavity. Therefore, the air fuel ratio around the ignition plug is undesirably small or the combustion mixture is undesirably made rich.
In addition, if the depth of the cavity (the height of a side wall of the cavity opposite to the injector) is too small, the injected fuel is carried over the top of the side wall of the cavity. Further, the fuel amount attached to a bottom surface of the cavity is increased so that the vaporization of the fuel is deteriorated. As a result, the air fuel ratio around the ignition plug is undesirably increased.